How the ichthyosaur got its fins

Reptiles first evolved on the land, but several groups later returned to the oceans, including the ichthyosaurs. The evolutionary origins of these top predators, some of them as large as today’s sperm whales, have long been unclear. But now researchers have found fossils of a small, semiaquatic reptile that may exemplify the group’s transition back to the sea.

Ichthyosaurs (which in Greek means “fish lizards”) lived from about 248 million years ago to about 95 million years ago, says Da-Yong Jiang, a vertebrate paleontologist at Peking University in Beijing. Species ranged in size from 1 meter to 20 meters long, and many of these streamlined, sharp-toothed creatures evolved to resemble dolphins and whales. Like modern-day marine mammals, which also returned to the sea from life on land, they were air-breathers.

The new fossil of the presumed proto-ichthyosaur was unearthed from rocks in eastern China in 2011. Those rocks were laid down as marine sediments about 248 million years ago, Jiang says. The well-preserved fossil is quite complete, with only a portion of the tail missing, he notes. When alive, the creature was probably about 40 centimeters long—about the size of a large lizard—and weighed about 2 kilograms.

The fossil’s vertebrae and ribs are thick-walled and well mineralized, so the creature was probably an adult when it died. But most bones in the forelimbs were small and widely separated, a sign that the forelimbs were likely cartilage-filled flippers and not legs, Jiang says. The creature’s hindlimbs were reduced in size, another adaptation for life at sea. The new species’ name, Cartorhynchus lenticarpus, comes from the Greek words for “short snout” (another feature of the fossil) and Latin words for “flexible wrist.” The creature’s heavy bones, like a scuba diver’s weight belt, counteracted natural buoyancy and probably enabled it to forage in shallow coastal waters, Jiang says. Yet C. lenticarpus could probably get around well on land using its flippers, just as seals and sea lions do today. Jiang and his colleagues describe the new species online today in Nature.

C. lenticarpus “is the closest thing we have to a terrestrial ancestor” of ichthyosaurs, says Valentin Fischer, a vertebrate paleontologist at the University of Liège in Belgium, who wasn’t connected to the research. He first saw reports of the fossils at a meeting of paleontologists a couple of years ago. Afterward, he notes, “I stood up and thought, ‘This is going to be huge.’ ”

Although C. lenticarpus is on the way to an aquatic lifestyle, Fischer notes, its head and snout aren’t fully streamlined, and its flexible flippers may have been more useful for getting around on land than for propulsion during swimming. Nevertheless, he suggests, the creature could comfortably get around in terrestrial and aquatic environments.

C. lenticarpus evolved just a few million years after the end-of-Permian mass extinctions, which wiped out as many as 90% of the species on land and 70% of those in the oceans. Therefore, Fischer says, many ecological niches were vacant and ready for evolution to fill them with new and interesting creatures. Reptiles returning to the seas were just one such experiment, and C. lenticarpus was just one species in that gradual evolution.

“There are more ancestral forms [of ichthyosaurs] still to be found,” Fischer notes. Unearthing such species, he adds, could help paleontologists identify the terrestrial creatures from which ichthyosaurs evolved.

Yet C. lenticarpus, a creature with obvious aquatic specializations, “might destroy the once-popular idea that we’re destined to find semiterrestrial ‘walking proto-ichthyosaurs,’ ” says Darren Naish, a vertebrate paleontologist at the University of Southampton in the United Kingdom, who also wasn’t connected to the study. “We won’t find them because ichthyosaurs and their kin emerged from a group that was already strongly aquatic.”